1. Field of the Invention
The present invention relates to a display device and a method of fabricating a display device, and more particularly, to a field sequential liquid crystal display device and method of fabricating a field sequential liquid crystal display device.
2. Discussion of the Related Art
Cathode-ray tube (CRT) devices have been commonly used for visual display systems. However, development of flat panel display devices are increasingly being used because of their small depth dimensions, desirably low weight, and low power consumption. Currently, thin film transistor-liquid crystal display (TFT-LCD) devices have been developed having high resolution and small depth dimensions.
In general, a liquid crystal display (LCD) device includes an upper substrate, a lower substrate, and a liquid crystal material layer interposed therebetween. The upper and lower substrates each have electrodes opposing one another. When an electric field is supplied to the electrodes of the upper and lower substrates, molecules of the liquid crystal material layer become aligned according to the applied electric field. By controlling the electric field, the liquid crystal display device provides various light transmittances to display images. Accordingly, an active matrix liquid crystal display (AM-LCD) device commonly used because of its high resolution and superior display of moving images. An active matrix liquid crystal display has a plurality of switching elements and pixel electrodes that are arranged in an array matrix configuration on the lower substrate. Accordingly, the lower substrate of the active matrix liquid crystal display is commonly referred to as an array substrate.
FIG. 1 is a cross sectional view of a liquid crystal display device according to the related art. In FIG. 1, a liquid crystal display includes a liquid crystal panel 10 and a backlight device 60, wherein the liquid crystal panel 10 includes a color filter substrate (i.e., an upper substrate) 20 and an array substrate (i.e., a lower substrate) 30 that face each other across a liquid crystal material layer 50. In addition, the color filter substrate 20 includes a color filter layer 22 and a black matrix 26 formed on a rear surface of a transparent substrate 1. The color filter layer 22 includes one of red (R), green (G), and blue (B) color filters, and the black matrix 26 is disposed among the red (R), green (G) and blue (B) color filters for preventing light leakage. A common electrode 24 is formed on a rear surface of the color filter 22 to function as one of an electrode pair for applying an electric field to the liquid crystal material layer 50.
The lower substrate 30 includes a thin film transistor T, which functions as a switching element, formed on the transparent substrate 1 to face the upper substrate 20. A pixel electrode 34, which is electrically connected to the thin film transistor T and functions as a second one of the electrode pair for applying the electric field to the liquid crystal material layer 50, is formed on the transparent substrate 1 of the array substrate 30. First and second polarizers 25 and 35 are formed on outer surfaces of the transparent substrate 1.
The backlight device 60 is disposed under the array substrate 30 to irradiate light to the liquid crystal panel 10. The back light 60 includes a white light source 62 to emit white light along a direction to the liquid crystal panel 10. Although not shown in FIG. 1, the thin film transistor T includes a gate electrode, a source electrode, and a drain electrode. The liquid crystal display panel 10 supplies a voltage to the pixel electrode 34 via the thin film transistor T, wherein the electric field between the pixel electrode 34 and the common electrode 24 rearranges an alignment direction of the liquid crystal molecules of the liquid crystal material layer 50. The white light emitted by the backlight device 60 is transmitted through the liquid crystal panel 10 having the color filters 22 to display color images. Due to a polarization of white light and optical anisotropy of liquid crystal molecules, the polarized light is modulated by passing through the red (R), green (G), and blue (B) color filters, thereby producing color images.
Although not shown in FIG. 1, the upper and lower substrates 20 and 30 are attached to each other by a seal pattern formed along peripheries of the upper and lower substrates 20 and 30. To align the liquid crystal molecules along a desired direction, upper and lower alignment layers (not shown) are provided between the liquid crystal material layer 50 and the upper substrate 20 and between the liquid crystal layer 50 and the lower substrate 30, respectively.
However, the active matrix liquid crystal display device in FIG. 1 has significant problems. First, since the transmissivity of a material used for forming the color filters is less than 33%, a brighter backlight device is required in order to effectively display the color images. Accordingly, the active matrix liquid crystal display device requires increased power consumption. Second, since the material used for forming the color filters is expensive, manufacturing costs increase. In addition, as a thickness of the color filters increase in order to improve saturation and chromaticity of the displayed color images, the transmissivity of the liquid crystal panel is reduced. On the contrary, if the thickness of the color filters decreases to improve the transmissivity, the displayed color images will have poor degrees of resolution.
As a result, field sequential liquid crystal display (FS LCD) devices, which display full color images without using the color filters, have been developed. The active matrix liquid crystal display devices display the color images by constantly transmitting the white light from the backlight device to the liquid crystal panel, whereas the field sequential liquid crystal display devices display the color images by sequentially and periodically turning ON and OFF the light sources, which have Red (R), Green (G), and Blue (B) colors.
FIG. 2 is a schematic cross sectional view of one pixel region of a field sequential liquid crystal display device according to the related art, and FIG. 3 is a schematic block diagram of a field sequential liquid crystal display device according to the related art. Since the same reference numbers may be used for the same parts in both FIGS. 2 and 3, some explanations may be omitted to prevent duplication.
In FIGS. 2 and 3, the field sequential liquid crystal display device includes a circuit unit 80, a liquid crystal display panel 10, and a backlight device 61. The circuit unit 80 receives RGB data and other control signals from an external driving system 70 (i.e., a computer system) and controls the received data and signals. The liquid crystal display panel 10 displays images by aligning and rearranging liquid crystal molecules, and the backlight device 61 irradiates light to the liquid crystal display panel 10.
In FIG. 2, the liquid crystal display panel 10 includes an upper substrate 20 and a lower substrate 30 that face each other across a liquid crystal material layer 50. The upper substrate 20 includes a black matrix 26 formed on a rear surface of a transparent substrate 1. Unlike the liquid crystal display device of FIG. 1, the color filter layer is not disposed on the upper substrate 20. In addition, a transparent common electrode 24 is formed on the rear surface of the transparent substrate 1 to cover the black matrix 26.
In FIG. 2, the lower substrate 30 includes a thin film transistor T, which functions as a switching element, formed on the transparent substrate 1 to face the upper substrate 20. A pixel electrode 34, which is electrically connected to the thin film transistor T and serves as a first electrode for applying an electric field to the liquid crystal material layer 50, is formed on the transparent substrate 1 of the array substrate 30. First and second polarizers 25 and 35 are formed on outer surfaces of the transparent substrates 1, respectively. In addition, the backlight device 61 includes three light sources Red (R) 64, Green (G) 66, and Blue (B) 68 to irradiate colored light to the liquid crystal display panel 10.
In FIG. 3, the liquid crystal display panel 10 includes a plurality of data lines 36 and gate lines 38 that perpendicularly cross each other to define a plurality of pixel regions P in a matrix configuration. The plurality of data lines 36 are formed in parallel to one another and the plurality of gate lines 38 are formed in parallel to one another, wherein both the data and gate lines 36 and 38 are disposed between the upper and lower substrates 20 and 30. Within each of the pixel regions P, the thin film transistor T is disposed as a switching element, and a liquid crystal capacitor CLC and a storage capacitor CST are disposed within each of the pixel regions P. The pixel electrode 34 and the common electrode 24 constitute the liquid crystal capacitor CLC, and the storage capacitor CST is connected in parallel with the liquid crystal capacitor CLC in order to solve parasitic capacitor problems.
One of the most significant differences between the field sequential liquid crystal display devices of FIGS. 2 and 3 and the liquid crystal display of FIG. 1 is that the field sequential liquid crystal display devices does not require the color filters in the upper substrate 20 and the backlight device 61 including three different light sources 64, 66, and 68 that are sequentially and selectively turned ON and/or OFF. The three different light sources 64, 66, and 68 are each driven by an inverter (not shown) and each are sequentially turned ON and OFF in one frame of one-sixtieth ( 1/60) of a second. In the field sequential liquid crystal display device, one frame of 1/60 of a second is divided into three sub-frames each being one-hundred-eightieth of a second ( 1/180 second) of a period. During each sub-frame, the liquid crystal molecules of the liquid crystal material layer 50 are rearranged, and then one of the light sources 64, 66, and 68 is turned ON and OFF. Thus, during one frame, the rearrangement of the liquid crystal molecules and the enablement of one of the red, green, and blue light sources are sequentially repeated.
In FIG. 3, the circuit unit 80 includes an interface 82, a timing controller 84, a gamma generating unit 86, a data driver 88, and a gate driver 90, wherein the circuit unit 80 controls and changes the RGB data and other control signals originating from the driving system 70 into desired signals in order to enable the liquid crystal panel display 10 to display the color images. The interface 82 directly receives the RGB data and other control signals from the driving system 70, and delivers the data and signals to the timing controller 84.
The control signals include a plurality of timing synchronization signals such that the timing controller 84 receiving the timing synchronization signals generates data control signals and gate control signals, respectively. Thus, the data control signals are supplied to the data driver 88 for driving the data driver 88, and the gate control signals are supplied to the gate driver 90 for driving the gate driver 90.
In addition, the timing controller 84 transmits the RGB data received from the interface 82 to the data driver 88. The gamma generating unit 86 generates an RGB reference voltage using the RGB data and transmits the RGB reference voltage to the data driver 88. Accordingly, the RGB reference voltage is set by the intrinsic transmissivity-voltage characteristics of the liquid crystal display panel 10.
The data driver 88 supplies an RGB image voltage, which controls the alignment direction of the liquid crystal molecules, to each of the data lines 36 using the RGB reference voltage transmitted from the gamma generating unit 86. The gate driver 90 supplies a scanning signal voltage, which turns the thin film transistor T ON and OFF, to each of the gate lines 38 using the gate control signals. When the thin film transistor T of a selected pixel region P is turned ON, the RGB image voltage is transmitted to the liquid crystal capacitor CLC.
If the R, G, and B light sources 64, 66, and 68 are sequentially turned ON and OFF in an order of R-G-B, the interface 82 receives the R data and its control signal from the driving system 70 during the first sub-frame. Those R data and control signal are transmitted to the timing controller 84 and inverted to the data and gate control signals for driving the data and gate drivers 88 and 90. Then, the gamma generating unit 86 outputs the R reference voltage using the R data, and the data driver 88 supplies the R image voltage to all of the data lines 36. Accordingly, the gate driver 90 outputs the scanning signal voltage sequentially from the G1 gate line to the Gm gate line using the gate control signals, thereby rearranging the direction of the liquid crystal molecules of the liquid crystal material layer 50 within the selected pixel regions P. The rearrangement of the selected pixel regions P corresponding the G1 gate line is maintained until the liquid crystal molecules of the pixel regions P corresponding to the Gm gate line are rearranged. After supplying the scanning signal voltage to all of the gate lines 38, the R light source 64 is turned ON to display the red (R) color image.
Accordingly, the second sub-frame handles the G data and its control signal through the above sequence, thereby displaying the G color image like the first sub-frame. The third sub-frame also handles the B data and its control signal, and thus displays the B color image. Accordingly, one frame is complete by way of sequentially conducting the first to third sub-frames.
Each of first to third sub-frames takes 1/180 seconds, and thus the single frame takes 1/60 seconds. Accordingly, a color image caused by the combination of three colors (red, green, and blue) is displayed using an afterimage (i.e., residual image) effect of human vision. Although the Red (R), Green (G), and Blue (B) light sources are turned ON and OFF one-hundred and eighty times per second, the perception by the naked eye is that the light sources are constantly ON due to the afterimage (or residual image) effect. For example, if the Red light source is turned ON and the Blue light source is sequentially turned ON, a mixed color (i.e., violet) is shown due to the residual image effect. Furthermore, if all of the R, G, and B color images show the lowest transmissivity, the human eye perceives a black color.
FIG. 4 is a graph illustrating a relationship between transmissivity and applied voltage in a field sequential liquid crystal display device using Optical Compensated Birefringent (OCB) mode according to the related art, and FIG. 5 is an enlarged view of portion A of the graph in FIG. 4 according to the related art. In FIGS. 4 and 5, transmissivity differences are noticeable depending on the R, G, and B colors although the same reference voltage is applied. In FIG. 5, since the R, G, and B colors have different wavelengths, the R, G, and B colors have different lowest transmissivities such that the combination will produce the black color. In particular, the B color wavelength arrives to the lowest transmissivity earlier than the R and G color wavelengths, and the B color wavelength has a relatively large transmissivity as compared to the R and G colors around the voltage necessary to produce the black color. In addition, the B color wavelength is generated during displaying the black color, thereby producing the blue shift phenomenon. Not only in the OCB mode but in the other modes, the transmissivity difference appears under the same voltage in the OCB mode and in other modes, and a color shift may be generated that degrades the image quality of the liquid crystal display.